Add digital circuit section

[namberino]

I found that a lot of the computer science curriculum does teach about digital circuits and I think knowledge about digital circuits can become extremely useful when working with computers and logic.

I'd like to contribute to this project by adding a digital circuit section to this project. This section will contain lessons about basic digital circuits like AND gate, NAND gate, OR gate, etc, and more complex digital circuit like flip-flops, adders, latches, etc. This section could later be developed to include more complex digital circuits like ALU, RAM, control unit, etc.

I believe this section can complement the boolean logic section very well. I believe this will be very useful for those who wants to work in embedded systems and IoT, which is a growing field. This can also come in handy for those who wants to work on chip and IC design, a blooming field.

I'd love to work on this. When it's complete, I'll submit a pull request for review

[namberino]

@shhossain can you assign this to me please?